In the press forming of the metal plate, spring-back deformation is caused in a pressed product released from the mold due to elastic recovery, which is a problem for dimensional accuracy of a product. Recently, the use of thin-walled, high-tensile steel sheets or aluminum alloy plates is particularly increasing in automobile constructional products for achieving weight reduction of a vehicle body. These materials are large in the spring-back deformation so that defect of the dimensional accuracy is actualized.
The defect of the dimensional accuracy in the cross-section of the product includes an angle change of a vertical wall, a wall warp, and distortion, bending and camber-back as a whole of the product. There have been proposed a great number of countermeasures against each of these defect phenomena. The camber-back is a bad phenomenon that a curved shape in a product having a curve in its longitudinal direction is returned back after press forming, or a radius of curvature is made larger as compared to a target radius of curvature. The camber back is significantly caused in products being low in the wall height and small in the section stiffness.
As a countermeasure for the camber back in a product having a hat cross-sectional shape, for example, JP-A-2013-063462 discloses a technique of suppressing the camber back by making the height of a vertical wall smaller than a difference between a radius of curvature in a hat head portion and a radius of curvature in a flange portion. However, that technique cannot be applied to all of product shapes because there is a restriction in the product shape.
Also, JP-A-2010-207906 discloses a technique of suppressing the camber back by deforming a top portion of an arc-like curved part into a concave shape in an anti-bulging direction under pressure on the way of the press forming to decrease residual tensile stress in the top portion. In that technique, however, the concave shape is formed in the top portion of the arc-like curved part as shown in examples so that the product shape is restricted, but also a shape-correcting process such as subsequent restriking step is required when the concave shape is unnecessary.
In the conventional techniques, it is obliged to restrict the product shape to suppress the camber back as mentioned above. Therefore, it could be helpful to provide a press forming method capable of suppressing camber back in products having a hat or channel cross-sectional shape and a longitudinally curved shape viewing from a vertical wall direction (from a side) and a method of manufacturing a pressed product as well as a press forming apparatus, or a press forming mold used in these methods.